Encoding of binary digital data signals for transmission of secret messages is well-known, many special code sequences being used to secure message secrecy. Most systems that pulse code modulate (PCM) secret messages utilize generators of binary sequences that modulate-demodulate the message to be transmitted-received. One generally used generator of binary sequences for coding is a pseudo-random noise generator of pseudo-random (PN) codes--see the E. M. Kartchner, et al., U.S. Pat. No. 3,665,472 for a discussion of such PN codes. In the E. Angeleri, et al., U.S. Pat. No. 3,659,046 there is disclosed a system for scrambling-unscrambling a binary digital data signal using PN codes for data signal security. The present invention is believed to be an improvement over such prior art systems in which it is able to provide greater coding combinations with fewer elements.